While an absolutely great game, Nintendo's Wii mentality has taken a bit away from Mario Kart Wii.
Nintendo definitely casualized the game. All their other hardcore games like Zelda, Metroid, Mario, and Super Smash Bros they made more approachable for the likes of the new casual audience they've obtained with the wii, but all their core gameplay elements remained mostly unchanged. With Mario Kart Wii, they've totally made it a casualized affair. Mario Kart used to be able to be played easily by those who picked it up and played but there was a sense of depth where hardcore players could excel in the game. Now with changes such as easy powersliding made it so skill is not even a major factor in each game. The thing you will be relying most on (especially on 150cc) is luck. With 12 characters in each race, theres that many more weapons and you'll be subjected to a cluster of god knows what hitting you at every turn. One more negative is the Wii wheel. It really isn't that bad, but if you're a vet player, you'll be finding yourself using the likes of a gamecube controller or other alternate method more than the Wii wheel.
Bad aside, this game has some great things going for it. First off is the sheer amount of courses and characters. There are 32 courses in the game and 24 or 25 characters. Of the 32 courses, 16 of them are remakes from previous Kart games. Some of them are welcomed classics, some are "eh" much like some of the "eh" new characters like Funky Kong... really? Funky Kong? Who cares? Also new to the series are motorcycles. The motorcycles don't really handle that differently than the karts, but there is definitely fun to be had with them.
Another big addition to this game is online, which is highly functional for a Wii game. You'll be able to go up against 11 other combatants in races or battles. 12 player multiplayer is definitely a huge bump up from the fun four player races that you're used to, which are still there and there's nothing like player Mario Kart with 3 of your friends all on the same TV. Also intergrated with the online portion of the game is the Mario Kart Channel which you can download to the Wii menu. This channel allows you to view stats, download ghosts, post ghosts, manage friends, ect. It's a really cool feature which I hope is implemented more in future games.
Over all, Mario Kart is a great game. If you've played Mario Kart before you'll be at home despite the casual approach to the game, and if you're new to the series, it's a great time to start. But like I said, the casual approach to the game definitely takes away from it and is surprisingly one of the few times Nintendo has shot themselves in the foot with their decisions this time.